Melt bittersweet chocolate bar (broken into pieces) in top of double boiler over hot water.

Beat butter and sugars together on medium-high speed until light and fluffy. Turn mixer down to lowest speed, add eggs and vanilla. In separate bowl, sift together flour, baking powder, baking soda, salt, and espresso powder. Add half of the flour mixture to creamed butter, stirring after each addition. Add half of the melted chocolate. Stir until ingredients are well blended.